1. Field of Invention
The present invention relates to a technology for forming a thin layer of toner on a developing roller that supplies toner to a photosensitive drum.
2. Description of Related Art
A typical laser printer that uses a non-magnetic single-component toner has a developing roller that carries thereon the toner, a layer thickness-regulating blade disposed at a side of the developing roller for forming a thin toner layer on the developing roller, and a photosensitive drum disposed facing the developing roller, on which drum an electrostatic latent image is formed.
The layer thickness-regulating blade is pressed against a surface of the developing roller. Toner supplied onto the developing roller is rubbed between the layer thickness-regulating blade and the developing roller, so that toner is sufficiently electrically charged and, at the same time, is placed as a thin layer of a predetermined thickness on the developing roller. An electrostatic latent image is formed on the photosensitive roller by uniformly electrically charging the surface of the photosensitive roller through the use of an electrical charging device and then irradiating the surface with a laser beam based on predetermined image data through the use of a laser emitting device. Thus-electrically charged toner is transferred selectively to the electrostatic latent image on the photosensitive roller, thereby developing the image.
However, to develop an image of a toner of a single non-magnetic component, the electrical charging of toner is carried out substantially entirely by friction contact between the layer thickness-regulating blade and the developing roller. Therefore, to uniformly charge the toner, it is necessary to quite strongly rub the toner by strongly pressing the layer thickness-regulating blade against the developing roller. In order to improve the flowability and the stability of the electrical charge, an external additive made of fine powder of silica, alumina, titanium oxide or the like is added to a toner. Therefore, if the toner is rubbed excessively strongly, particles of the external additive become sunk in toner matrix particles, so that the flowability decreases or the electrical charging characteristic of toner decreases.